Well logs are measurements, typically with respect to depth, of selected physical parameters of earth formations penetrated by a wellbore. Well logs are typically recorded by inserting various types of measurement instruments into a wellbore, moving the instruments along the wellbore, and recording the measurements made by the instruments. One type of well log recording includes lowering the instruments at the end of an armored electrical cable, and recording the measurements made with respect to the length of the cable extended into the wellbore. Depth within the wellbore is inferred from the extended length of the cable. Recordings made in this way are substantially directly correlated to measurement depth within the wellbore.
Other methods for measurement include a “logging while drilling” (LWD) method, a measurement while drilling (MWD), and a memory logging method. The LWD method involves attaching the instruments to the lower portion of a drilling tool assembly used to drill the wellbore. LWD and wireline tools are typically used to measure the same sort of formation properties, such as density, resistivity, gamma ray, neutron porosity, etc. MWD tools are typically used to measure parameters closely associated with drilling, such as well deviation, well azimuth, weight-on-bit, mud flowrate, annular borehole pressure, etc.
The aforementioned well logging tools may be conveyed into and out of a well via wireline cable, drilling pipe, coiled tubing, slickline, etc. Further, LWD and MWD measurement methods allow for measurement in the drill string while the bit is cutting, or measurement while tripping down or up past a section of a borehole that had been drilled at a previous time.
Some measurement tools use a pressure modulation telemetry system, which modulates pressure of a drilling fluid (mud) flowing through the interior of the drilling tool assembly, to obtain well log data. However, a much larger quantity of well log data is stored in a recording device disposed in the log instrument, which is interrogated when the instrument is retrieved from the wellbore. This information is typically recorded with respect to time. A record of instrument position in the wellbore with respect to time made at the earth's surface is then correlated to the time/measurement record retrieved from the instrument storage device to generate a conventional “well log” of measurements with respect to wellbore depth.
Well logs are typically presented in a graphic form including a plurality of grids or “tracks” each of which is scaled from a selected lower value to a selected upper value for each measurement type presented in the particular track. A “depth track” or scale, which indicates depth in the wellbore, is typically positioned between two of the tracks. Depending on the needs of the particular user, any number of or type of measurements may be presented in one or more of the tracks. A typical well log presentation of an individual measurement is in the form of a substantially continuous curve or trace. Curves are interpolated from discrete measurement values stored with respect to time and/or depth in a computer or computer-readable storage medium. Other presentations include gray scale or color scale interpolations of selected measurement types to produce the equivalent of a visual image of the wellbore wall. Such “image” presentations have proven useful in certain types of geologic analysis.
Interpreting well log data includes correlation or other use of a very large amount of ancillary information. Such ancillary information includes the geographic location of the wellbore, geologic and well log information from adjacent wellbores, and a priori geological/petrophysical knowledge about the formations. Other information includes the types of instruments used, their mechanical configuration and records relating to their calibration and maintenance. Still other types of information include the actual trajectory of the wellbore, which may traverse a substantial geographic distance in the horizontal plane with respect to the surface location of the wellbore. Other information of use in interpreting well log data includes data about the progress of the drilling of the wellbore, the type of drilling fluid used in the wellbore, and environmental corrections applicable to the particular log instruments used.
Much of this ancillary information is applicable to any well log recorded with a particular type of well log instrument. For example, an instrument, which measures naturally occurring gamma radiation (“gamma ray”), has environmental corrections, which correspond only to the type of instrument. As one example, each wireline type gamma ray device of a selected external diameter from a particular wireline operator has the same environmental corrections for “mud weight” (drilling fluid density).
Other types of ancillary information are made available from the wellbore operator (typically an oil and gas producing entity). Examples of this type of information include the geographic location of the wellbore and any information from other wellbores in the vicinity. Still other types of ancillary information include records of initial and periodic calibration and maintenance of the particular instruments used in a particular wellbore. The foregoing is only a small subset of the types of ancillary information, which may be used in interpreting a particular well log.
FIG. 1 shows a typical manner in which well log data are acquired by “wireline,” wherein an assembly or “string” of well log instruments (including sensors or “sondes” (8, 5, 6 and 3) as will be further explained) is lowered into a wellbore (32) drilled through the earth (36) at one end of an armored electrical cable (33). The cable (33) is extended into and withdrawn from the wellbore (32) by means of a winch (11) or similar conveyance known in the art. The cable (33) transmits electrical power to the instruments (8, 5, 6, 3) in the string, and communicates signals corresponding to measurements made by the instruments (8, 5, 6, 3) in the string to a recording unit (7) at the earth's surface. The recording unit (7) includes a device (not shown) to measure the extended length of the cable (33). Depth of the instruments (8, 5, 6, 3) within the wellbore (32) is inferred from the extended cable length. The recording unit (7) includes equipment (not shown separately) of types well known in the art for making a record with respect to depth of the instruments (sensors) (8, 5, 6, 3) within the wellbore (32).
The sensors (8, 5, 6, and 3) may be of any type well known in the art for purposes of the invention. These include gamma ray sensors, neutron porosity sensors, electromagnetic induction resistivity sensors, nuclear magnetic resonance sensors, and gamma-gamma (bulk) density sensors. Some sensors, such as (8, 5, and 6) are contained in a sonde “mandrel” (axially extended cylinder) which may operate effectively near the center of the wellbore (32) or displaced toward the side of the wellbore (32). Others sensors, such as a density sensor (3), include a sensor pad (17) disposed to one side of the sensor housing (13) and have one or more detecting devices (14) therein. In some cases, the sensor (3) includes a radiation source (18) to activate the formations (36) proximate the wellbore (32). Such sensors are typically responsive to a selected zone (9) to one side of the wellbore (32). The sensor (30) may also include a caliper arm (15), which serves both to displace the sensor (30) laterally to the side of the wellbore (32) and to measure an apparent internal diameter of the wellbore (32).
The instrument configuration shown in FIG. 1 is only meant to illustrate in general terms acquiring “well log” data by “wireline” and those skilled in the art will appreciate alternatives and modifications that are equally applicable.
FIG. 2 shows a typical configuration for acquiring well log data using a logging while drilling (LWD) and measurements while drilling (MWD) system (39). The LWD/MWD system (39) may include one or more collar sections (44, 42, 40, 38) coupled to the lower end of a drill pipe (20). The LWD/MWD system (39) includes a drill bit (45) at the bottom end to drill the wellbore (32) through the earth (36). In this example, drilling is performed by rotating the drill pipe (20) by means of a rotary table (43). However, drilling may also be performed by top drives and coiled tubing drilling with downhole motors. During rotation, the pipe (20) is suspended by equipment on a drill rig (10) including a swivel (24), which enables the pipe (20) to rotate while maintaining a fluid tight seal between the interior and exterior of the pipe (20). Mud pumps (30) draw drilling fluid (“mud”) (26) from a tank or pit (28) and pump the mud (26) through the interior of the pipe (20), down through the LWD/MWD system (39), as indicated by arrow (41). The mud (26) passes through orifices (not shown) in the bit (45) to lubricate and cool the bit (45), and to lift drill cuttings in through an annulus (34) between the pipe (20), LWD/MWD system (39), and the wellbore (32).
The collar sections (44, 42, 40, 38) include sensors (not shown) therein which make measurements of various properties of the earth formations (36) through which the wellbore (32) is drilled. These measurements are typically recorded in a recording device (not shown) disposed in one or more of the collar sections (44, 42, 40, 38). LWD systems known in the art typically include one or more sensors (not show), which measure formation properties such as density, resistivity, gamma ray, neutron porosity, etc. as described above. MWD systems known in the art typically include one or more sensors (not show) which measure selected drilling parameters, such as inclination and azimuthal trajectory of the wellbore (32). MWD systems also provide the telemetry (communication system) for any MWD/LWD tool sensors in the drill string. Other drilling sensors known in the art may include axial force (weight) applied to the LWD/MWD system (39), and shock and vibration sensors.
The LWD/MWD system (39) typically includes a mud pressure modulator (not shown separately) in one of the collar sections (44). The modulator (not shown) applies a telemetry signal to the flow of mud (26) inside the system (39) and pipe (20) where the telemetry signal is detected by a pressure sensor (31) disposed in the mud flow system. The pressure sensor (31) is coupled to detection equipment (not shown) in the surface recording system (7A), which enables recovery and recording of information transmitted in the telemetry scheme sent by the MWD portion of the LWD/MWD system (39). As explained, the telemetry scheme includes a subset of measurements made by the various sensors (not shown separately) in the LWD/MWD system (39). The telemetry of the logging tools may also be determined using wireline cable (not shown), electrical MWD telemetry (i.e., using electrical signals transmitted through the formation), or wired drill pipe. The remainder of the measurements made by the sensors (not shown) in the LWD/MWD system (39) may be transferred to the surface recording system (7A) when the LWD/MWD system (39) is withdrawn from the wellbore.
In a similar manner to the wireline acquisition method and system shown in FIG. 1, the LWD/MWD acquisition system and method shown in FIG. 2 is only meant to serve as an example of how data are acquired using MWD/LWD systems, and those skilled in the art will appreciate alternatives and modifications that are equally applicable.
Well log formats have been the traditional method for displaying multiple passes of formation and borehole measurements. A typical well log data presentation is shown in FIG. 3. The data presentation shown in FIG. 3 is typically made substantially entirely from data recorded by the well log instrument and entered in the recording system by an operator at the wellsite. As described above, the well log data are typically presented on a grid-type scale including a plurality of data tracks (50, 54, 56). The presentation shown in FIG. 3 is a standard format prescribed in, Standard Practice 31A, published by the American Petroleum Institute, Washington, D.C. which includes 3 such tracks. The tracks (50, 54, 56) include a header (57) which indicates the data type(s) for which a curve or curves, (51, 53, 55, 59) are presented in each track. A depth track (52), which shows the measured depth (or alternative depth measure such as true vertical depth) of the data is disposed laterally between the first (50) and second (54) data tracks. The depth tracks (52) may alternatively use a time-based scale. Data curves (51, 52, 53, 54) are presented in each of the tracks (50, 54, 56) corresponding to the information shown in the header (57). The example data presentation of FIG. 3 is only one example of data presentations that may be used with embodiments of the invention and those skilled in the art will appreciate alternatives and modifications that are equally applicable.
A presentation such as shown in FIG. 3 may include in the various curves (51, 53, 55, 59) “raw” data, such as values of voltages, detector counts, etc. actually recorded by the various sensors in the well log instrument (not shown in FIG. 3), or more commonly, shows values recorded by the sensors converted to values of a parameter of interest, such as natural gamma radiation level, resistivity, acoustic travel time, etc. These presentations may generally be made only from the raw data themselves and universally applied scaling and correction factors. Still other presentations of the various curves may include data to which environmental corrections have been applied. Typically, raw data and such minimally corrected data may be recorded at the wellsite without the need to enter significant amounts of data other than the data recordings from the instruments themselves.
Two files are used to routinely process and present LWD data, as shown in FIG. 3, a time-depth file and a time-data file. The time-depth file contains a column of clock time and a column of bit depth (i.e., the depth of the bit at the corresponding clock time). The time-data file contains a column of clock time (synchronized with the time-depth file) and a corresponding sensor measurement ( e.g., a resistivity measurement) made at that clock time. With a known depth offset between the bit and the measurement sensor, the measurement value can be displayed at the corresponding formation depth. Typically, only the first measurement at a specific depth is displayed. All subsequent sensor measurements at a specific depth are disregarded in standard LWD log processing, regardless of how many more times the sensor passes by that depth. An abundance of potentially valuable data subsequently goes unused and unseen.
FIG. 4 shows a textual representation of a typical time-depth file. The time is displayed in first column (400) in an hh:mm:ss format. The bit depth corresponding to the time is displayed in a second column (402). The bit depth is recorded at 10-second intervals. In section (404) the bit depth is increasing, in section (406) the bit depth is decreasing (i.e., certain portions of the wellbore are passed by a second time), in section (408) the bit depth continues to increase, in section (410) the bit depth decreases, and in section (412) the bit depth subsequently increases.
As shown in FIG. 4, the bit depth does not always get deeper; sometimes in the process of drilling a well, the bit (and hence everything in the drillstring) is raised “off bottom” of the wellbore. As the drill string is raised and subsequently lowered back to “bottom,” the bit and measurement sensors in the drillstring, pass by the same depth interval of the formation. These subsequent trips past the same depth interval can be seconds, minutes, hours, days, and/or weeks apart.
FIG. 5 shows a textual representation of a time-data file corresponding to the time interval shown in FIG. 4. The time is displayed in first column (500) using an hh:mm:ss format. The sensor measurement, in this example resistivity corresponding to the time, is displayed in a second column (502). The sensor measurement is recorded at 10-second intervals. Traditional processing of the time-depth file (FIG. 4) and the time-data file (FIG. 5) typically presents sensor measurements whenever the bit depth increases. Upon close inspection, the traditional resistivity log, using the time-depth file shown in FIG. 4 and the time-data file shown in FIG. 5, indicates that the formation resistivity is 1.0 Ω-m from 1000 ft to 1022 ft. However, every time the sensor made another pass over the same depth interval, the sensor recorded a value of 1.5 Ω-m. For example, at a depth of 1009 ft, the first pass of the sensor recorded a resistivity of 1.0 Ω-m, and a second pass of the sensor at 1009 ft recorded a resistivity of 1.5 Ω-m. The overlapping data (i.e., sensor measurements of the same depth at different times) is traditionally unseen and unused. Yet, a change in a sensor measurement, between the two passes, may indicate a change in downhole conditions, such as formation and borehole conditions, that may be useful in determining how an operator should proceed in drilling, completing, or producing a well. In the example shown in FIGS. 4 and 5, a change in the resistivity value may provide an early indication that the formation is fracturing, or it may help determine the type of fluid within the pore space.